Electromechanical oscillator using electret coupling

ABSTRACT

An electrically driven oscillator device is provided with one or more electrets for establishing electrostatic coupling between a mechanical oscillating element and an electrical drive. The mechanical oscillating element can comprise a reed, a tuning fork, a pendulum, or a hair-spring balance wheel. The electrical drive can be an AC voltage source or an electronic feedback circuit. In electrically driven devices, electrets can be used in both the pickup and driving circuits to eliminate hysteresis. Alternatively, in order to isolate the pickup circuit from the driving circuit, electrets can be used in only one of these circuits and magnetic arrangements can be used in the other.

States ateni Cruger et a1.

[ 51 Mar. 28, N72

[54] ELECTROMECHANICAL OSCILLATOR USING ELECTRET COUPLING I [73]Assignee: General Time Corporation, Phoenix, Ariz.

[22] Filed: July 30, 1970 [21] Appl. No.: 59,653

[52] U.S.Cl .331/1 16M,58/23AO,58/23TF,

[51] Int. Cl. ..H03b 5/30 [58] Field of Search ..331/ll6M,154,156;58/23,

58/23 A, 23 A0, 23 TF, 2s A; 84/409; 318/128, 1. 3 12 3 2 52 17 13, ZL193112211 1 E [56] References Cited UNITED STATES PATENTS 3,243,9514/1966 Kawakami 58/23 v 3,118,022 1/1964 Sessler et a1. ..179/181 X3,515,914 6/1970 Steinemann ..58/23 TF 3,350,667 10/1967 Shreve ..333/712,292,790 8/1942 Millar ..331/156 Primary Examiner-Roy Lake AssistantExaminer-Siegfried H. Grimm Attorney-Pennie, Edmonds, Morton, Taylor andAdams [5 7] ABSTRACT An electrically driven oscillator device isprovided with one or more electrets for establishing electrostaticcoupling between a mechanical oscillating element and an electricaldrive. The mechanical oscillating element can comprise a reed, a tuningfork, a pendulum, or a hair-spring balance wheel. The electrical drivecan be an AC voltage source or an electronic feedback circuit. 1nelectrically driven devices, electrets can be used in both the pickupand driving circuits to eliminate hysteresis. Alternatively, in order toisolate the pickup circuit from the driving circuit, electrets can beused in only one of these circuits and magnetic arrangements can be usedin the other.

14 Claims, 12 Drawing Figures PATENTED MR 2 8 1912 SHEET 1 OF 3 FIG 222| zoz $5 2 219 I 2|o 220 208 206 am 5.5. 55- 207 0 rl 0 I 0 L IMF-4%w-aoo 222% ms 302 2n AMPLIFIER RI 2|4 INVENTORS RICHARD CRUGER BY WILMERC. ANDERSON ROBERT W. WINDEBANK DAVID EARLS ATTORNFYS PATENTED RZBIHIZ3,652,955

'SHEET 2 BF 3 2" AMPLIFIER DRAIN SUBSTRATE SOURCE .JI- T 400 FIG. 4C

507 INVIZNTORS 0 RICHARD CRUGER 8 ARR? II IIIIISA IPIIK B B 508 Y DAVIDRLS M, I 2 2 g; I

503 50! 509B ATTORNEYS PATENTEDHAR28 r972 3,652,955

sum 3 OF 3 v AMPLIFIER VOLTAGE 7 AMPLIFIER THRESHOLD A VipTAGE A ANGLEOF CLOCKWISE ROTATION VOLTAGE AMPLIFIER THRESHOLD F G. VOLTAGE ANGLE OFCOUNTER- CLOCKWISE ROTATION INVENTORS RICHARD CRUGER WILMER C. ANDERSONBY ROBERT W. WINDEBANK DAVID EARLS ATTORNEYS ELECTROMECIIANICALOSCILLATOR USING ELECTRET COUPLING BACKGROUND OF THE INVENTION Thepresent invention relates to an electrically driven oscillatory devicewhich utilizes one or more electrets to establish electrostatic couplingbetween a mechanical oscillating element and an electrical drive.

Electrically driven oscillatory devices are useful in a variety ofapplications such as timekeeping devices and high-Q oscillators. Suchdevices typically comprise a mechanical element capable of periodicoscillation which is magnetically coupled to an electrical drive byinteracting electromagnets and permanent magnets. The oscillatingelement can comprise any one of a number of known mechanical elements,such as a reed (U.S. Pat. No. 2,036,917 issued to M. P. Favre-Bull, Mar.8, 1935), a tuning fork (British Pat. No. 155,854 issued to W. H. Eccleset al., Jan. 6, 1921), a pendulum (French Pat. No. 1,092,411 publishedApr. 21, 1955), or a hairspring balance wheel (U.S. Pat. No. 2,769,946issued to H. D. Brailsford, Nov. 6, 1956). The electrical drive cancomprise an AC source, a transistor amplifier, or vacuum-tube amplifier.

A typical AC driven oscillatory motor is illustrated by the F avre-Bulldevice which comprises, in essence, a spring reed fixed at one end and apermanent magnet attached at the other end. The reed is mounted so thatone pole of the magnet is disposed between the two poles of a curvedelectromagnet. When an AC current is applied to the electromagnet, thereed is driven back and forth as the polarity of the electromagnetalternates.

A typical electronic oscillatory motor is illustrated by the devicedisclosed in U.S. Pat. No. 2,971,323 issued to M. Hetzel, Feb. 14, 1961.The Hetzel device comprises a tuning fork having a pair of permanentmagnets attached to its tines for interacting with separate pickup anddriving coils. The pickup coil is connected to the input terminals of atransistor amplifier, and the driving coils are connected to theamplifier output terminals. In operation, the vibrations of the forkmove one of the magnets into the pickup coil and induce a voltage whichis applied to the input terminals. When the input voltage becomessufficiently large, it triggers a current pulse which passes through thedrive coil and augments the vibration of the fork.

A number of problems arise because of the use of magnetic couplingbetween the mechanical oscillating element and the electrical drive. Onesuch problem is hysteresis and eddy currents which can introduce lossand frequency distortion into the system. 1n order to reduce loss, it isoften necessary to use laminated steel cores, such as are shown inFavre-Bull and Brailsford; in order to avoid undesirable distortion,such as phase shifting between the pickup and the driving coils in anelectronically driven motor, the coupling arrangements and the circuitelements must be very carefully designed and put together. (Theseproblems are discussed in greater detail in U.S. Pat. Nos. 2,950,447 and2,034,787 issued to C. H. McShan and A. J. Williams, Jr., respectively.)Another difficulty which arises in such electronically drivenoscillatory devices is that of spurious self-induced electricaloscillations which occur at the frequency determined by the inductanceand the distributed capacity of the coils rather than at the naturalfrequency of the mechanical oscillatory element. This problem isdiscussed in detail in the previously cited McShan reference.

While attempts have been previously made to eliminate the use ofmagnetic coupling by providing an oscillatory motor with electrostaticcoupling between the oscillating element and the electrical drive, theseattempts have generally proved impractical or unsatisfactory for manyapplications. For example, U.S. Pat. No. 2,934,887 issued to R. Kelleron May 3, 1960, describes a hairspring balance wheel oscillatory motorin which the oscillating balance wheel is electrically charged due tothe emission of alpha or beta radiation from a coating of radioactiveisotope, such as Co, C, Sr, Pu One difficulty with such a motor,however, is the potential health hazard that the radioactive isotopesmay present. For example, the fabrication of such motors is difficultbecause of the elaborate safety precautions which must be taken in thebandling of radioactive isotopes. Moreover, the motors may presenthealth hazards to nearby persons as, for example, the wearer of a watchemploying such a motor. Another example of an electrostatic motorpurporting to eliminate magnetic losses is described by J. Favey in U.S.Pat. No. 2,835,105 issued May 20, 1958. To achieve the high voltagerequired for this motor, however, a transformer is employed in the drivecircuit; and, as is well known, transformers typically produce loss anddistortion due to hysteresis and eddy currents.

SUMMARY OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWINGS The nature,features, and advantages of the invention will appear more fully uponconsideration of the various illustrative embodiments now to bedescribed in detail in connection with the accompanying drawings.

IN THE DRAWINGS:

FIG. 1 is a schematic cross section of an AC driven oscillating reed inaccordance with the invention;

FIG. 2 is a schematic cross section of an electronically driven tuningfork in accordance with the invention;

FIG. 3 is a schematic cross section of an electronically drivenpendulum;

FIGS. 4A, 4B, and 4C illustrate a first example of a balance wheeloscillator device in accordance with the invention;

FIGS. 5A and 5B illustrate a second balance wheel device in accordancewith the invention which is adapted to utilize both electrostaticattraction and repulsion in driving the wheel;

FIGS. 6A and 6B illustrate yet another example of an electronicallydriven balance wheel;

FIG. 7 is a graphical illustration useful in understanding theembodiment of the invention shown in FIG. 6.

DETAILED DESCRIPTION In reference to the drawings, FIG. 1 illustrates anoscillating reed device comprising an oscillating reed 100, such as ablade of spring steel, having one end fixed to a stationary base 101 andthe other end attached to a pair of electrets 102 and 103. In the reedrest position, the electrets are disposed midway between a pair ofelectrodes 104 and 105 which, in turn, are electrically coupled to asource 106 of AC voltage, such as an outlet for volt, 60 cycle persecond line voltage. Means, such as the well-known pawl and ratchetwheel arrangement, are provided for converting oscillatory motion of thereed into rotary motion. Specifically, a pawl 107 tipped with a hardstone, such as ruby or sapphire, is attached to reed 100 and disposed toengage the teeth of a ratchet wheel 108. Advantageously, a pair of stops109 and 110, such as plastic covered metal stops, are fastened atsuitable positions on a stationary frame (not shown) for limiting theamplitude of the reed and preventing the electrets from contacting theelectrodes.

Electrets, as is well known in the art, are dielectric materials inwhich a permanent state of electrostatic polarization has beenestablished by the displacement of the positive charges in constituentatoms with reference to negative charges. They were first described byHeaviside in 1892. Electrets are typically produced by subjecting adielectric material, such as Mylar, polycarbonate, orpolytetrafiuoroethylene to heat in the presence of a high electricfield. The dielectric is then permitted'to cool while the field isapplied. The properties of and the methods for fabricating electrets areexplained in detail in the literature of the art. See, for example, U.S.Pat. No. 3,1 18,022 issued to G. M. Dessler on Jan. 14, 1964.

In operation, the application of an AC voltage across electrodes 104 and105 drives the reed back and forth as the electrodes change polarity.This oscillatory motion is converted into rotary motion by the pawl andratchet wheel arrangement and can be used, for example, to drive thegear train (not shown) of an electric clock.

The primary advantage of this device over typical prior art structuresis its simplicity. No curved electromagnets with wound coils of copperwire or heavy laminated steel cores are required. In addition, thelosses and distortion associated with hysteresis and eddy currents areavoided.

FIG. 2 shows an example of an electronically driven tuning fork inaccordance with the invention comprising, in essence, atuning forkcoupled to both the input terminals of an amplifier and the outputterminals, wherein at least one of the coupling arrangements includesone or more electrets. In the embodiment of FIG. 2, both the input andthe output terminals are coupled to the fork by electret arrangements.More specifically, a tuning fork 201 having tines 202 and 203 is rigidlyattached by its base 204 to a stationary baseplate 205. Tine 202 of thefork is provided with electret means, such as a pair of electrets 206and 207 attached to an extension 208 affixed to the tine, forelectrostatically coupling the tine to an electronic drive circuit. Thisis accomplished with a pair of pickup electrodes 209 and 210 disposedadjacent to the electrets at the fork rest position. A resistor R, isplaced in parallel with the pickup electrodes to permit current flowbetween the electrodes. The pickup voltage signal is applied to theinput terminals 211 and 212 of electronic amplifier 213. The amplifiercan comprise an NPN-transistor T, a DC voltage source V and a resistor Rin a common base arrangement. As will be appreciated by those skilled inthe art, a wide variety of other amplifying arrangements, such as thoseusing vacuum tubes or MOSFET devices can also be used.

The amplifier output terminals 214 and 215 are electrically connected toa pair of drive electrodes 216 and 217 disposed adjacent to a secondpair of electrets 210 and 219 which are attached to an extension 220affixed to tine 203. The polarities of the electrets and the electrodesare chosen and adapted to provide a drive signal of suitable polarityfor sustaining oscillations of the fork. As in the device of FIG. 1, apawl 221 and a ratchet wheel 222 are provided for convertingoscillations of the fork into useful rotary motion.

In operation, the vibration of tine 202 moves electrets 206 and 207 backand forth with respect to electrodes 209 and 210. This motion producesan alternating voltage across resistor R which is applied to the inputterminals of amplifier 213. Advantageously, amplifier 213 is chosen oradapted to have a minimum threshold voltage so that the pickup voltagesignal triggers either an attracting drive pulse to the drive electrodesjust before electrets 218 and 219 reach the position of alignment withthe drive electrodes or a repelling drive pulse just as they pass thisposition. (In even more sophisticated embodiments the circuitry can bedesigned to provide both types of driving pulses, as will be shownbelow.) In the example shown attracting pulses are applied to the driveelectrodes. In this manner, electrical energy from the amplifier is usedto sustain oscillations in the fork.

The primary advantage of this device is the elimination of the loss anddistortion due to hysteresis and eddy currents. A secondary advantage isthe savings in cost due to the elimination of the coils used in priorart devices.

FIG. 3 shows a schematic side view of an electronically driven pendulumin accordance with the invention comprising a rod 300 pivotally mountedat one end to a support member 301 and attached to a transverse member302 at the other end. The transverse member includes electret meanscomprising, for example, a pair of oppositely oriented electrets 303 and304 for coupling to a pair of pickup electrodes 305 and 306 and anotherpair of electrets 307 and 308 for coupling to a pair of drive electrodes309 and 310 for electrostatically coupling the pendulum to an electronicdrive circuit. The electrets, pickup electrodes, and drive electrodesare advantageously positioned so that the electrodes and electretsoverlap at the pendulum rest position. The electronic circuitry for thisdevice is substantially the same as that described in connection withFIG. 2.

In operation, the movement of electrets 303 and 3041 through the pickupelectrodes 305 and 306 induces alternat ing voltage pulse acrossresistor R, applied to the input terminals of amplifier 213. The outputof the amplifier, in turn, is applied to the drive electrodes 309 and310 to sustain oscillations in the pendulum. The oscillatory motion ofthe pendulum is converted into useful rotary motion by the pawl andratchet wheel arrangement, and stops are provided for maintaining theamplitude of the pendulum within useful limits.

FIGS. 4A and 4B illustrate, respectively, perspective and cross sectionviews of a first example of a balance wheel oscillatory device inaccordance with the invention. This device comprises, in essence, abalance wheel oscillator including electret means for coupling thebalance wheel to an electronic drive circuit. In particular, the devicecomprises a balance wheel 400 rigidly attached to a rotatably mountedshaft 401 and elastically attached to at least one of a pair of fixedmembers (stators) 402 and 403 by a hairspring 404. Stator 102 isprovided with a pickup electrode 405, and stator 403 is provided with adrive electrode 406.

Balance wheel 4100 is provided with electret means comprising pickupelectret 4107 and drive electret 408 for electrostatically coupling withelectrodes 405 and 406, respectively. It is also provided withconductive means which act as a common ground electrode with respect tothe pickup and drive electrodes. In particular, the balance wheel isconveniently made of a conductive material such as a metal and isprovided with electrical contact means such as terminal 409 attached toa metal hairspring 404.

Shaft 401 extends through stator 402 and is mechanically coupled tomeans, such as gear 410, for turning a ratchet wheel.

The electronic drive circuitry is substantially the same as thatdescribed previously. It comprises, in essence, an electronic amplifierpreferably comprising a metal-oxide-silicon field effect transistor(henceforth MOSFET) having its input terminals electrically connectedbetween balance wheel 400 and pickup electrode 405 (which acts as thepickup electrode). The amplifier output is connected between the balancewheel and electrode 406, and electret 400 is appropriately driven by theoutput voltage.

FIG. 1C illustrates the preferred circuit arrangement for using anN-channel MOSFET however, as is easily seen, a P- channel MOSFET can beused with only minor circuit modifications.

FIG. 5A shows a schematic cross section of a second balance wheel devicein accordance with the invention which is especially adapted to utilizeboth electrostatic attraction and repulsion in driving the wheel. Thedevice is substantially the same as that shown in FIGS. 4A and 413except that the balance wheel is provided with an electrically separatereference electrode for the pickup and the drive electrets. This isaccomplished by using a balance wheel comprising a pair of metal discs500A and 50013 mechanically connected by an insulator 500C. In addition,separate electrical contacts 509A and 50913 are conveniently provided byuse of a pair of hairsprings 5041A and 5048. The remaining componentsare substantially identical to their counterparts previously describedin connection with FIG. -1.

The electronic drive circuit for use with this embodiment of theinvention is chosen or adapted to supply an attracting voltage pulse tothe drive electrode as the drive electret ap proaches the point ofmaximum overlap and a repelling voltage as it passes this point. Inparticular, the pickup electrode is electrically connected to the inputterminals of two amplifying means, one such amplifying means forapplying an attracting pulse to the drive electrode as the pickupelectret approaches the pickup electrode and the other amplifying meansfor applying a repelling pulse as the pickup electret passed the pickupelectrode. (The pickup and the drive electrets are, of course,positioned so that they each approach their respective electrodes at thesame time.) A preferred embodiment of such a drive circuit is shown inFIG. 5B. In this circuit the amplifying means for applying theattracting pulse comprises a pair of N-channel MOSFETS T and T and theamplifying means for applying the repelling pulse comprises a pair ofP-channel MOSFETS T and T Yet another example of an electronicallydriven balance wheel is shown in schematic perspective and cross sectionviews in FIGS. 6A and 68, respectively. In this arrangement, an electretpickup arrangement is used in conjunction with a magnetic drivearrangement. In addition, a specially shaped electrode or electret isused to prevent activation of the magnetic driving arrangement againstthe motion of the oscillator.

ln specific reference to the figures, the device comprises a balancewheel 600 rigidly affixed to a rotatably mounted shaft 601 andelastically attached to a stator 602 by a hairspring 603. The balancewheel is provided with electret means comprising electret 604 forelectrostatically interacting with pickup electrode 605 attached to thestator and is also provided with permanent magnet means comprising apair of magnets 609 and 607 for magnetically interacting with anelectromagnet 608 attached to the stator. The electronic circuitry usedwith this device is simply an amplifier arrangement adapted to deliver acurrent pulse to electromagnet 608 for driving the balance wheel throughthe permanent magnets is response to an above-threshold voltage from thepickup electrode 605.

Since, however, the electret produces a pickup voltage on the electrodein both directions of oscillation, the electromagnet will drive inopposition to the balance wheel motion on a return (counterclockwiserotation) unless some corrective means are provided. In this embodimentof the invention, the electrode (or electret) is shaped so that an abovethreshold voltage is produced during the forward oscillation but notduring the return oscillation. This is accomplished by making theelectrode roughly triangular shaped with a vertex pointing in thedirection of forward oscillation. Since the voltage induced is afunction of the rate of change of capacity with respect to time, aforward (clockwise) oscillation of the wheel produces a relatively largepositive voltage pulse due to the abrupt increase in capacity as theelectret suddenly begins to interact with the base of the triangularelectrode and a negative voltage of low magnitude as the electret movesfrom the base to vertex. This waveform is graphically shown in FIG. 7A.However, on the return trip, the situation is reversed; the positivevoltage remains relatively small because the time rate of change ofcapacity is relatively small and constant as the electret moves from thevertex to the base, and the negative pulse is relatively large as theelectret moves over the base and away from the electrode. This waveformis shown in FIG. 7B. Consequently, the arrangement can be designed toselectively trigger a drive current pulse during only one direction ofoscillation.

In all cases, it is understood that the above-described examples areonly illustrative of the many possible specific embodiments which canrepresent applications of the principle of the invention. Numerous andvaried other arrangements can be made by those skilled in the artwithout departing from the spirit and scope of the invention.

We claim:

I. An oscillatory device comprising:

a mechanical element capable of periodic oscillation;

means for applying electrical energy for sustaining periodicoscillations of said mechanical element; and

electret means for electrostatically coupling said mechanical elementand said means for applying electrical energy.

2. A device according to claimv 1 including means for producing rotarymotion from oscillatory motion of said mechanical element.

3. A device according to claim 2 wherein:

said means for applying electrical energy comprises a pair of electrodesfor receiving an alternating voltage; and

said electret means is attached to said mechanical element and disposedbetween said electrodes.

4. A device according to claim 3 wherein said mechanical element is areed.

5. A device according to claim 2 herein:

said means for applying electrical energy for sustaining periodicoscillations comprises a pickup arrangement responsive to the motion ofsaid mechanical element, amplifying means responsive to the signal fromsaid pickup arrangement for producing an electrical driving signal, anda driving arrangement for sustaining oscillation of said mechanicalelement.

6. A device according to claim 5 wherein:

said pickup arrangement comprises at least one pickup electrode; and

said electret mean for electrostatically coupling said mechanicalelement and said means for applying electrical energy comprises one ormore electrets attached to said mechanical element for electrostaticallyinteracting with said pickup arrangement.

7. A device according to claim 6 wherein said mechanical element is atuning fork.

8. A device according to claim 6 wherein said mechanical element is apendulum.

9. A device according to claim 5 wherein:

said driving arrangement comprises at least one driving electrode; and

said electret means for electrostatically coupling said mechanicalelement and said means for applying electrical energy includes electretmeans attached to said mechanical element for interacting with saiddriving arrangement.

10. A device according to claim 5 wherein:

said driving arrangement comprises electromagnet means;

and

said mechanical element includes permanent magnet means for interactingwith said electromagnet.

11. A device according to claim 5 wherein said mechanical element is abalance wheel elastically coupled to at least one stator member.

12. A device according to claim 9 wherein said mechanical element is abalance wheel elastically coupled to at least one stator member.

13. A device according to claim 10 wherein said mechanical element is abalance wheel elastically coupled to at least one stator member.

14. A device according to claim 13 wherein:

the amplifying means responsive to the signal from said pickuparrangement has a threshold voltage; and

the pickup arrangement is designed to produce an abovethreshold signalduring only one direction of oscillation.

l 0* i i

1. An oscillatory device comprising: a mechanical element capable ofperiodic oscillation; means for applying electrical energy forsustaining periodic oscillations of said mechanical element; andelectret means for electrostatically coupling said mechanical elementand said means for applying electrical energy.
 2. A device according toclaim 1 including means for producing rotary motion from oscillatorymotion of said mechanical element.
 3. A device according to claim 2wherein: said means for applying electrical energy comprises a pair ofelectrodes for receiving an alternating voltage; and said electret meansis attached to said mechanical element and disposed between saidelectrodes.
 4. A device according to claim 3 wherein said mechanicalelement is a reed.
 5. A device according to claim 2 wherein: said meansfor applying electrical energy for sustaining periodic oscillationscomprises a pick-up arrangement responsive to the motion of saidmechanical element, amplifying means responsive to the signal from saidpick-up arrangement for producing an electrical driving signal, and adriving arrangement for sustaining oscillation of said mechanicalelement.
 6. A device according to claim 5 wherein: said pick-uparrangement comprises at least one pick-up electrode; and said electretmeans for electrostatically coupling said mechanical element and saidmeans for applying electrical energy comprises one or more electretsattached to said mechanical element for electrostatically interactingwith said pick-up arraNgement.
 7. A device according to claim 6 whereinsaid mechanical element is a tuning fork.
 8. A device according to claim6 wherein said mechanical element is a pendulum.
 9. A device accordingto claim 5 wherein: said driving arrangement comprises at least onedriving electrode; and said electret means for electrostaticallycoupling said mechanical element and said means for applying electricalenergy includes electret means attached to said mechanical element forinteracting with said driving arrangement.
 10. A device according toclaim 5 wherein: said driving arrangement comprises electromagnet means;and said mechanical element includes permanent magnet means forinteracting with said electromagnet.
 11. A device according to claim 5wherein said mechanical element is a balance wheel elastically coupledto at least one stator member.
 12. A device according to claim 9 whereinsaid mechanical element is a balance wheel elastically coupled to atleast one stator member.
 13. A device according to claim 10 wherein saidmechanical element is a balance wheel elastically coupled to at leastone stator member.
 14. A device according to claim 13 wherein: theamplifying means responsive to the signal from said pick-up arrangementhas a threshold voltage; and the pick-up arrangement is designed toproduce an above-threshold signal during only one direction ofoscillation.